Radiative forcing components. Graphic: IPCC 2007 / Wikipedia

By Gayathri Vaidyanathan
23 April 2015

(Scientific American) – Slivers of dust float in the upper atmosphere, scattering the sun's rays back into space and cooling the planet in some places. In other places, the particles warm the planet.

The equivocation has meant that the particles, known as aerosols, are a significant wild card in our planet's climate, rivaled only by clouds. So it was arguably not surprising that a study on aerosols would receive public attention.

But it was not the type of attention that the study author, Bjorn Stevens, a climatologist and director at the Max Planck Institute for Meteorology in Hamburg, Germany, was seeking. His work has been portrayed by conservative news outlets and blogs as undermining the theory of human-caused global warming. Red lights lit up. "New Climate Paper Gives Global Warming Alarmists 'One Helluva Beating,'" Fox News declared.

In the months since the study was published, Stevens has been peppered with emails from schoolteachers and laypeople asking him, broadly speaking, whether climate change is indeed something to worry about. That brought the normally reticent scientist, who says his aim is not to convince anyone of anything, into the public sphere.

"I was touched that they'd write me and double-check that my study was being interpreted correctly," Stevens said, speaking on a train en route to the Netherlands.

The study in question, published in Journal of Climate, is titled rather drably, "Rethinking the lower bound on aerosol radiative forcing."

It delves into the impacts of aerosols, which are tiny pollutants of mineral dust, soot and organic matter emitted by sources such as power plants, factories, and quarries. Not to be outdone, nature occasionally spews her own aerosols from volcanoes.

The particles gather at the highest reaches of the atmosphere, where the net result is that they cool the planet. In the process, they somewhat mask the warming caused by greenhouse gases. So scientists have long harbored a fear: Perhaps aerosols are cooling the planet so much that in their absence, global temperatures will rise rapidly. Such a future may play out as nations curb pollution from industries. […]

The misinterpretation of Stevens' paper began with Nic Lewis, an independent climate scientist. In a blog post for Climate Audit, a prominent climate skeptic blog, he used Stevens' study to suggest that as CO2 levels double in the atmosphere, global temperatures would rise by only 1.2 to 1.8 degrees Celsius. The measure is called "climate sensitivity."

That's less than the assumed 2 C threshold for catastrophic climatic change in parts of the world. It's also lower than an IPCC estimate that a doubling of CO2 will raise global temperatures by 1.5 to 4.5 C.

Lewis' blog post prompted conservative publications to crow that global warming is not a major threat. Stevens was inundated with email.

"All sorts of schoolteachers were contacting me, and they were all worried that everything they'd learned was wrong," he said.  [more]

How to Misinterpret Climate Change Research


ABSTRACT: Based on research showing that in the case of a strong aerosol forcing, this forcing establishes itself early in the historical record, a simple model is constructed to explore the implications of a strongly negative aerosol forcing on the early (pre 1950) part of the instrumental record. This model, which contains terms representing both aerosol-radiation and aerosol-cloud interactions well represents the known time history of aerosol radiative forcing, as well as the effect of the natural state on the strength of aerosol forcing. Model parameters, randomly drawn to represent uncertainty in understanding, demonstrates that a forcing more negative than −1.0 W m−2 is implausible, as it implies that none of the approximately 0.3 K temperature rise between 1850 and 1950 can be attributed to northern-hemispheric forcing. The individual terms of the model are interpreted in light of comprehensive modeling, constraints from observations, and physical understanding, to provide further support for the less negative ( −1.0 W m−2 ) lower bound. These findings suggest that aerosol radiative forcing is less negative and more certain than is commonly believed.

Rethinking the lower bound on aerosol radiative forcing

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